The present invention relates to a structure of a semiconductor apparatus having semiconductor circuits made of semiconductor devices such as insulated gate transistors, and also to a method of manufacture thereof. Particularly, the invention relates to a technology that forms a crystalline semiconductor film over an insulating surface. The semiconductor apparatus include not only semiconductor devices such as thin-film transistors (TFTs) and MOS transistors but also displays and electrooptic apparatus, such as image sensors, both of which have semiconductor circuits made of insulated gate transistors. In addition, the semiconductor apparatus of this invention also include electronic apparatus incorporating these displays and electrooptic apparatus.
An active matrix liquid crystal display, which forms a pixel matrix circuit and a drive circuit by using thin-film transistors (TFTs) formed on an insulating substrate, is attracting attention. A liquid crystal display currently used as a monitor has a size range of 0.5–20 inches.
TFTs that use as an active layer a crystalline semiconductor film represented by polysilicon are being spotlighted as a means for realizing the liquid crystal display capable of displaying finely defined images. Although TFTs using a crystalline semiconductor film as an active layer have a faster operating speed and a higher driving capability than those TFTs that use an amorphous semiconductor film as an active layer, their TFT characteristics are difficult to control.
One of the causes for the difficulty in controlling the TFT characteristics is the property of an interface between the active layer and the insulating film. This interface, when contaminated, makes it difficult to manufacture the semiconductor devices with good controllability of the TFT characteristics. It is therefore important to clean the interface between the active layer and the insulating film.
Currently, there are growing demands on the TFTs for high mobility and it is considered more promising to use a crystalline semiconductor film with high mobility than to use an amorphous semiconductor film as the TFT's active layer. The method of manufacturing a conventional top gate type TFT using a crystalline semiconductor film will be briefly explained.
First, a substrate having an insulating surface is formed with a base insulating film (hereinafter referred to as a base film) and then heat-treated, after which it is deposited with an amorphous silicon film. Next, the amorphous silicon film is subjected to crystallization processing such as heating and laser beam irradiation to form a polysilicon film (polycrystalline silicon film). Next, the polysilicon film is patterned to a desired shape and deposited with an insulating film (gate insulating layer) and a conductive film (gate line forming material layer). Then these films are patterned to form gate lines. Next, impurities of p- or n-type conductivity are selectively introduced into the polysilicon film to form impurity regions, such as source and drain regions. This is followed by depositing an interlayer insulating film, forming contact holes to expose the source and drain regions, forming a metal film, and then patterning the metal film to form metal lines in contact with the source and drain regions. In this way, the process of manufacturing the TFTs is completed.
In the conventional technology described above, when the base film after being formed is subjected to the heat treatment to improve the TFT reliability, the surface of the base film is exposed to the atmosphere. At this time, the base film surface is contaminated with impurities contained in the air (boron, oxygen, water, sodium, etc.). If a semiconductor film that constitutes an active layer is formed over and in contact with the base film contaminated by the open air, the characteristic of an interface between the active layer, particularly a channel forming region, and the base film deteriorates, leading to a degradation in the electrical characteristic of TFTs.
Because the air in the clean room contains boron from a HEPA filter generally used for cleaning, an unspecified amount of boron mixes into the surface of the film exposed to the atmosphere. The HEPA filter is made of a mesh of glass used to remove minute particles in the air. The glass contains a large amount of boron to make it easy to manufacture the mesh-like structure. Using other filters than the HEPA filter is disadvantageous from the standpoint of reducing the manufacturing cost.
To investigate the effect of the impurities in the air, the insulating base film after being formed is exposed to the atmosphere and then deposited with a laminated structure of semiconductor films made of amorphous silicon films to form TFTs. The SIMS analysis on the manufactured TFTs revealed a concentration peak of boron whose maximum value was 3×1017 atoms/cm3. When boron mixes into the active layer of semiconductor film, it is diffused and activated by the processes that follow (heat treatment and laser beam processing, etc.) making the control of the impurity concentrations in the active layer difficult. Measurement of the TFT electric characteristic revealed a phenomenon in which the threshold voltage shifts to the plus side.
When impurities (boron, oxygen, water, sodium, etc.) enter into the active layer of semiconductor film, it is found that the crystallization of the semiconductor film is hindered.
The present invention provides a semiconductor apparatus having semiconductor circuits made of semiconductor devices which improve the interface between an active layer, particularly a channel forming region, and a base film to improve the TFT characteristic (such as threshold voltage) and which have high reliability. The invention also provides a method of manufacturing such semiconductor apparatus.